A double-pane window is not one thick piece of glass. It is two panes held apart by a spacer around the perimeter and sealed at the edge, with the space between them isolated from the air in your house and the air outside. When that edge seal fails, humid air gets into that space. As the glass heats and cools through the day, moisture in that trapped air condenses on the inner surfaces, and you see it as fog, haze, or streaking.
The important detail is where the moisture sits. It is on the two glass surfaces facing each other inside the assembly, which are not reachable from your living room or from your ladder outside. That is why washing the window changes nothing. People often clean a foggy window two or three times before concluding something is actually wrong with it, which is a completely reasonable thing to assume and also a waste of an afternoon.
Edge seals live a hard life. Every day the glass heats up and cools down, the assembly expands and contracts slightly, and that movement works the seal back and forth. Along the Wasatch Front, the spread between a hot summer afternoon and a cold winter night is wide, and that swing is hard on seals and frames both. Over enough cycles, the seal loses its grip somewhere along the edge, and once air can move in and out, moisture comes with it.
Sun load accelerates the whole process, which is why exposure matters so much. South and west facing windows take the most direct heat, so they cycle harder and further than the north side of the same house. If you walk your house and find that the fogged units are concentrated on one or two walls, you are looking at the exposure pattern rather than a manufacturing fluke, and the shaded windows of the same age are generally trailing behind rather than immune.
Not all window moisture means a failed seal, and the difference is easy to check. Put your finger on it. If the water is on the surface facing the room, that is condensation from indoor humidity meeting cold glass, and it wipes away. If it is on the outside face, that is dew or frost doing exactly what dew and frost do. Only moisture you cannot touch from either side is inside the sealed unit.
Frame material plays into this too. Older aluminum frames conduct temperature noticeably, so the frame itself gets cold and often collects water along the bottom rail on winter mornings. That is the frame behaving the way aluminum behaves, not a sign that the glass unit failed. It is worth knowing which one you are dealing with before you call, because one is about indoor humidity and frame material and the other is about a sealed unit that has already given out. They lead to completely different conversations about what to do next.
There are three general paths, and which one applies depends on what surrounds the glass. On some windows the sealed glass unit can be replaced while the sash and frame stay right where they are. On others the glass is not serviceable on its own, but the frame is sound, and replacing the sash is the sensible middle option. And when the frame is deteriorated or the hardware is worn out along with everything else, replacing the whole window is the honest answer.
What tips the decision is almost always frame and sill condition, plus how many units have failed. One hazy window in an otherwise solid house is a candidate for glass or sash work. Five hazy windows across two walls, in a house where the sashes stick and the locks no longer line up, is a set that has reached the end of its service life, and doing them one at a time will cost more attention and more money over the next few years.
A failed seal is not an emergency. Nothing is going to break, leak into your wall, or become unsafe because the glass looks hazy. What you are losing is the performance the unit was built to deliver and the ability to see out of it clearly, and it tends to look worse over time as moisture cycles through and leaves mineral deposits behind on the inner surfaces. Nobody needs to schedule this around a storm or a cold snap.
The practical approach is to walk the house once, list every window with haze, note which wall each one is on, and decide whether you want to handle them together or work through them by priority. Grouping the work into one visit is usually more efficient than calling someone out each time another window clouds over. It also gives you a clearer picture of whether you are maintaining a good set of windows or gradually replacing a tired one.
It can look like it does. On a dry, mild day the trapped moisture may evaporate enough that the glass looks clear again, then reappear when conditions change. That is not the seal healing, it is the same water moving between liquid and vapor inside the unit. Once air is getting in, the haze comes back, and it usually returns more often as time goes on.
It still has two panes and it is still better than a single piece of glass, but it is no longer the assembly it was designed to be. The sealed space that made it work has been compromised. Most people notice the appearance long before they notice any difference in comfort, which is why a fogged unit usually gets replaced for how it looks more than anything else.
Not necessarily on any schedule, but they are usually the same age, installed at the same time, and living in the same climate. Exposure decides the order. The south and west units generally go first, and the shaded sides follow later. Treat the first failure as information about the age of the whole set rather than as an isolated event.
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